This invention relates generally to a rail fastener and more particularly to an improvement in a direct fixation rail fastener for holding a rail onto a support structure which provides electrical isolation and vibration and sound attenuation between the rail and support structure.
Direct fixation rail fasteners have been employed extensively in recent years in place of tie-on ballast arrangements for affixing transit rails to rigid support structures. One known rail fastening system employs a rail plate shaped to accommodate the rail and a pad of elastomer sandwiched between the rail plate and the support structure. A pair of clips provide resilient clamping of the rail to the rail plate. The rail plate is fastened to the support structure through a pair of posts extending upwardly from the support structure through insulative bushings which allow limited vertical displacement of the rail plate with respect to the support structure as well as providing lateral adjustment of the rail with respect to the support structure. Examples of such rail fastenering systems are disclosed in U.S. Pat. Nos. 4,047,663 and 4,150,791.
Even though the rail fasteners disclosed in these patents accomplish these stated objects, the pad of elastomer at times exhibits a squirm which results from forces applied to the rail by the wheels of a rail-mounted vehicle. As the vehicle moves along a track, a differential pressure wave is caused to build up in the rail in front of the vehicle because of the leverage action which results from the localized vertical forces applied to the rail by the wheels of the vehicle. In addition to the differential pressure wave, the rail will be subjected to overturning moments and sheer forces particularly in a curved portion of the track. Prolonged squirming of the pad may allow the pad to creep out from underneath the rail plate resulting in decreased performance of the rail fastener.